1. Field of the Invention
The present invention relates generally to computer modems and communication devices and, more specifically, to a device that couples between a modem and a telephone line to prevent damage to the modem as a result of dangerous line conditions.
2. Description of the Related Art
A modem is a device that modulates data received from a computer so that it can be transmitted via a telephone line and demodulates data received from a telephone line so that it can be provided to a computer. A modem may be integrated internally into a computer or may be external in a separate enclosure and connected to a computer by a cable. To connect the modem to the telephone line, a user typically plugs one end of a cable into the modem and the other end into a telephone wall jack. Telephone wall jacks are commonly provided in homes, businesses, hotels and other buildings to provide access to the public telephone network. In the United States, wall jacks conform to the standard known as RJ-11. A RJ-11 jack has four pins that mate with four pins of a RJ-11 plug on the cable.
Plugging a modem into a wall jack can, under certain circumstances, cause damage to the modem. Many wall jacks used in businesses are not directly coupled to the public switched telephone network (PSTN), but rather are coupled to a private branch exchange (PBX) system, which is in turn coupled to the PSTN. A PBX system is owned by the business and provides telephone service to many extension lines in various locations, intercom service between extensions, and myriad other communication features. All modern PBX systems are digital. That is, some of the signals they provide at the wall jack corresponding to each extension are digital. The digital signals are used for controlling the operation of the digital telephone that is plugged into the wall jack. If a user plugs an ordinary modem directly into the wall jack of a PBX extension, the modem can be damaged by the digital signals, which typically provide current in excess of 500 milliamps (mA). Modems are designed to receive only the signals provided at the wall jack coupled directly to the PSTN, and those signals typically provide only tens of milliamps of loop current.
A modem can be damaged by other causes as well. Plugging a modem designed in accordance with the telephone line standards of one country into a wall jack in another country could damage it if the voltage, current or other parameters of the two countries'standard telephone line signals are incompatible. Modems can also be damaged by power surges that sometimes occur as a result of lightning striking a telephone line or other events.
Commercially available products exist that attempt to address some of the above-described problems. One such product is a line tester having warning indicator lights that illuminate if a user plugs it into a wall jack that provides current in excess of about 90 mA or that provides signals of polarity opposite that of the modem. Although most modems will operate regardless of the signal polarity of the telephone line, the operation of certain modems may be degraded if the modem polarity is opposite the telephone line polarity. Another such product is similar in that it has warning indicator lights that illuminate if a user plugs it into a wall jack that provides excessive current or that provides signals of polarity opposite that required by modems. In addition, it has another jack into which the user can plug the modem cable if the indicator light indicates safe conditions by remaining un-illuminated. It also has a switch that a user can toggle to reverse the polarity of the wall jack with respect to the polarity of the modem jack. It further has a surge-protection circuit that limits the current to the modem. Still another such product has a surge-protection circuit that maintains a constant loop current, a reversed-polarity indicator light, and a switch for reversing the polarity.
The above-mentioned products are inconvenient to use and require the user to perform a number of steps. Observing an indicator light and then plugging in a modem only if it remains un-illuminated requires some degree of understanding by the user of the excessive current problem and involves multiple steps. Similarly, observing an indicator light and then toggling a switch to reverse polarity requires some degree of understanding by the user of the polarity problem and involves multiple steps. Furthermore, the surge-protection circuits of these products may provide inadequate protection for the modem under certain circumstances. It would be desirable to provide a modem protection device that provides a higher level of protection and is easy to use.
These problems and deficiencies are clearly felt in the art and are satisfied by the present invention in the manner described below.